1. Field of the Invention
The present invention relates to gas occlusion-free and void-free, two-primary phase, solidifiable compounds, and derived void-free solidified composite materials, and more particularly to gas occlusion-free and void-free polymeric solidifiable compounds and derived void-free solidified composites, including methods and apparatus for producing same.
2. Description of Related Art
A. Terminology
Because certain terms in the field of the invention may be used in different ways to signify the same or slightly differing concepts, the following definitions are provided to promote clarity for the following description of the invention.
The term "primary solid phase" is defined herein as one or more distinct solid substances each physically homogeneous, in solid state, which serves primarily as material reinforcement upon solidification of the primary liquid phase.
The term "primary solidifiable liquid phase" is defined herein as one or more distinct liquid substances, each physically homogeneous, in liquid state, capable of solidifying to constitute a solid continuous material matrix that binds the primary solid phase at ambient temperatures.
The term "compound" is defined as the unsolidified state of a composite.
The term "composite" is defined here as any solid primary phase mixed with any primary solidifiable liquid phase, forming a monolithic two-phase solid state material upon solidification of the liquid primary phase.
When the two primary phases are mixed to yield a compound, these primary phases are no longer in their primary state but in an unsolidified, multiphase, "mixed state" as defined herein.
The term "voids" are defined herein as filled or unfilled spaces, within interstices of a packed primary solid phase or surface pores in solid constituents. Voids are further defined as gas phase occlusions within a primary liquid phase originating from entrainment and/or adsorption of air, water vapor and other gases within the interstices of the solids in the primary solid phase, within the primary solidifiable liquid phase or within the multiphase, mixed unsolidified state of the two phases. The term "voids", as defined above, specifically excludes intermolecular and atomic spaces, which are natural unfilled spaces in matter. Furthermore, the scale of physical measurement of voids herein is about one micron (10.sup.-6 m) or more.
B. Polymeric Compounds and Composites
An extremely wide range of products are being manufactured today from a specific class of two primary phase compounds in which the primary solidifiable liquid phase is a polymeric resin. The process leading to the production of a polymeric composite involves mixing a generic primary solid phase with a polymeric resin system, thereby constituting a two-phase unsolidified compound. Upon further processing, the polymeric resin in the mixed unsolidified state is made to solidify, or harden, in an appropriate forming device, such as a mold or a die, yielding a formed, solid composite with the shape, or configuration, of the forming device.
The role of the polymeric liquid resin system in polymeric composites is to provide an essential binding matrix to the primary solid phase upon solidification. Initially, its low viscosity provides an adequate liquid medium for mixing with the solids of the primary solid phase. Upon solidification, the resin matrix provides a continuous solid phase that enables the composite to behave monolithically as a single solid material body.
Resin systems in polymeric composites are further classified as either thermoplastic, which soften when heated and may be shaped or reshaped while in a semifluid state or thermosetting, which are generally low viscosity liquids that solidify through chemical cross-linking. The most common resin systems in polymeric composites are thermosetting, and the most predominant thermosetting resin is unsaturated polyester. Other thermosetting resins include epoxies, vinylesters, phenolics and urethanes.
Certain thermosetting polymer resin systems consist of solid polymer particles dissolved in a low viscosity liquid and solvent monomer, for example, an unsaturated polyester dissolved in monostyrene. The monomer plays the dual role of providing a solvent medium for the distribution of the polymer resin, and also has the ability to react with the polymer into a final solid state. Such thermosetting resin systems are made to harden or solidify into a permanent shape by an irreversible chemical reaction known as curing or cross-linking, in which linear polymer chains and monomer chains in the liquid resin system are joined, or reacted, together to form complex, highly rigid, three-dimensional solid structures. This reaction requires anaerobic conditions; i.e., the liquid resin system will not harden in the presence of air. Thus the presence of O.sub.2 is known to have an inhibitory effect on the polymerization/solidification process. Additionally, water, which is known to diffuse into liquid thermosetting resin systems, significantly impairs the cross linking solidification reaction.
An additional property of thermosets is that they are generally brittle. Thus, thermosets are rarely used without some form of solid reinforcement. However, high resistance to weight ratio, ability to solidify at ambient temperatures and retain their shape and properties at somewhat elevated temperature as, well as good creep resistance and corrosion resistance properties, give thermoset resin systems significant advantages over thermoplastics. These advantages essentially are the reasons for their preference in the developmental history of polymeric composites.
The role played by the solids in the primary solid phase matrix of polymeric composites is one of structural reinforcement. Moreover, the choice of geometrical shape of the solid phase constituents is a function of the intended reinforcement requirement of the particular polymeric composite in terms of the type of predominant stresses from externally applied forces that are to be resisted. The geometrical shape of the solid reinforcement generally can be of two generic classes: 1) filament shaped, or fiber and 2) granular/spherical shaped, or aggregate-type solid material. The fiber reinforced polymeric composites are intended for predominantly tensional, mechanical resistance applications, whereas the aggregate reinforced polymeric composites are intended for predominantly compressional, mechanical resistance applications. These generic classes of solids can be viewed as forming two ends of the structural resistance spectrum of polymeric composites.
Polymer composites composed of fibrous solid materials mixed with thermosetting polymeric resin are known as "Fiber Reinforced Polymers" or FRPs. The most common fibers used in the present art are glass, graphite, ceramic and polymeric fibers. Depending on the particular production process used, this generic class includes polymeric composite materials such as "Glass Reinforced Plastics" (GRP), produced by open, manual or spray, lay up methods, pultrusion, filament winding, etc. or by enclosed methods such as "Resin Transfer Molding" (RTM), Seeman Composites Resin Infusion Manufacturing Process (SCRIMP), etc. Other FRP composites produced by enclosed methods are based on polymeric compound materials, such as "Bulk Molding Compound" (BMC), "Sheet Molding Compound" (SMC), "Thick Molding Compound" (TMC), etc. In the mixed solidifiable compound state, the latter fiber reinforced polymeric materials, appropriately handled, can be stored for extended periods of time for future forming and curing at appropriate combinations of pressure and temperature into final solid composite products.
Solid aggregate materials mixed with thermosetting polymeric resin (resins) matrices comprise the generic class of polymeric composites known as cast polymer products, polymer concretes, polymer mortars or polymer grouts. To date, the inorganic aggregates for polymeric composites have not been systematically characterized, but most common aggregates used in the present art are siliceous. Silica aggregates are widely used in the production of polymer concretes due to their mechanical, dielectric and chemical resistance properties, as well as for their abundance and low cost.
Thermosetting polymeric composites offer inherent advantages over traditional materials (metals, cement concrete, wood, ceramics and natural inorganic materials), including energy efficiency, high strength-to-weight ratio, design flexibility, parts consolidation, corrosion resistance, high dielectric and thermal properties, excellent appearance, low maintenance and extended service life. A vast array of thermosetting polymeric composite products are currently available worldwide in over 50,000 s successful applications developed over the past 45 years. Well over 95% of the U.S. production is dedicated to fiber reinforced polymeric composites, and the industry's shipments and growth are tracked under nine major market segments totaling over 3.2 billion pounds per year. Aggregate polymeric composites are widely used as cast materials for bath tubs, shower stalls, kitchen sinks and counters, flooring and decorative panels in construction. Cast polymer concrete products find use in specialized niche industrial applications, where the combination of high structural strength, corrosion and dielectric resistance is required.
Despite some differences, these two generic classes of polymeric composites have much in common in terms of certain characteristics and general behavior. Generally, the functional concepts and behavioral aspects of the polymeric resin systems are the same for both classes of generic composites, despite specific differences in the properties and geometries of the solids within each class. Key common and inherent characteristics of polymeric composites include: 1) the composites are all heterogeneous and most are anisotropic; 2) the composites generally exhibit considerable variability in their properties compared to metals; this variability is directly related to the volume of the respective fractions of the two phases, i.e., the primary liquid phase versus the primary solid phase; and 3) the composites follow a general "rule of mixtures," in which a property of the composite is equal to the sum of solid and resin matrix properties weighted by their respective volume fractions. The rule of mixtures, however, is not valid for most properties in fiber reinforced polymeric composites, except for longitudinal extensional modulus. In aggregate reinforced polymeric composites, the correlation of properties determined by the "rule of mixtures" is reasonably valid for many properties and supports the art of solid filler additives, commonly used to enhance desired properties in the composite, and/or mitigate the effects of undesired properties.
Heterogeneity in a two-phase polymeric composite material refers to certain properties that vary from point-to-point throughout the mass of the material. In a random selection of a point inside the material, properties can be very different, depending on whether the chosen point falls in the polymeric matrix or in the solid component . While it is true that generally all composite materials are heterogeneous at the micron level, the degree of heterogeneity is generally more pronounced in fiber polymeric composites.
Additionally, the heterogeneity of these materials contributes to the significant variability of properties of thermosetting polymeric composites. In the case of FRPs, properties depend on the combination of several factors, such as the properties of the constituents, the form of the fiber reinforcement used (continuous fibers, woven fibers, chopped fibers, etc.), fiber volume fraction, length, distribution and orientation, bond strength between the phases, and void content. For example, strength and hardness characteristics of FRPs with continuous length fibers depend strongly on fiber orientation, spatial distribution and the variability of the properties of the specific fiber chosen. As it is impossible to position each fiber individually in the mix, the variability of the properties of the material is inevitable. The variability of composites reinforced with discontinuous fibers, such as bulk molding compounds (BMC) and sheet molding compounds (SMC) which are ultimately shape-molded and cured in closed dies, is even more enhanced due to the difficulty in controlling local uniformity of fiber content and orientation in the face of material flow. Accordingly, material hardness and strength in the finished fiber reinforced composites made of BMC or SMC may vary considerably from point-to-point throughout the material.
Anisotropy is another characteristic common to thermosetting polymeric composites, and is generally more pronounced in fiber polymeric composites than in aggregate polymeric composites. An anisotropic material is one whose properties vary with direction. In the case of FRPs with straight, parallel and continuous fibers, the strength of the material is significantly stronger and stiffer in the direction parallel to the fibers than in the transverse direction.
Reinforcing fibers used in fiber polymeric composites are man-made in continuous processes yielding fine filaments that are quite brittle, and generally consisting of diameters ranging from 2 to 13 microns. Filaments are normally in bundles of several strands as rovings or woven into fabrics. Glass fibers are the oldest, cheapest and most widely used. They have generally good chemical resistance, are noncombustible and do not adsorb water, although generally they adsorb humidity from air in atmospheric conditions. Their tensile strength-to-weight ratios are relatively high, with elastic moduli in the range of those of aluminum alloys. The internal structure of glass fibers is amorphous, i.e., noncrystaline, and are generally considered isotropic.
Reinforcing aggregates used in aggregate polymeric composites are natural occurring inorganic materials that require processing to remove undesirable contaminants, such as clays, iron oxides, etc. This processing involves mechanically sieving the granules, separating them by sizes, and drying them within 0.1% humidity by weight to assure compatibility with the resin systems. Humidity strongly affects interfacial bonding of the resin with a dramatic drop in compression and flexure strengths. Geological origin, impurity levels, particle size distribution, and particle shape all affect uniformity and homogeneity of dispersion of the aggregates in the liquid resin system. These factors influence, in turn, interfacial bond strength and void content. For high corrosion resistance, thoroughly washed and dried, high silica content aggregates are generally used. Rounded, spherical-shape aggregates generally provide better mechanical and physical properties than crushed, angular-edged aggregates, and also yield higher packing aggregate fractions with reduced void content and reduced resin fraction volume.
C. Voids in Polymeric Compounds and Composites
Voids are a major factor significantly contributing to property variation within a polymeric composite. Voids tend to reduce the integrity of the material and its mechanical and dielectric strengths, cause optical defects and lower the chemical resistance.
Any open space or volume in the surface of solid matter, or the interstices of fractured packed solids, exposed to atmosphere are subject to atmospheric air pressure, which will instantaneously fill these spaces with air. For example, when solid silica is fragmented and packed, as in the case of silica aggregates for polymeric concretes, or when filaments of molten silica glass are packed together to form glass fiber, as in the case of fiberglass, the mass of the fragmented packed aggregates or packed filaments exhibit an "apparent or bulk density" which is significantly lower than the respective unfragmented or unfilamented specific density of the respective original solid materials. For example, silica has a specific density of 2.65 g/cm.sup.3 whereas the same silica fragmented into small diameter particles, approximately from say 100 microns up to say 6 mm, has a "bulk" density of only 1.6 g/cm.sup.3. This "bulk density" indicates that the silica particles of irregular geometries in contact with each other, as when packed in a heap, leave random dimensional interstices or spaces--voids that are filled with air. Neglecting the weight of air, the sum of voids in one cubic centimeter of particulated silica is equivalent to the volume occupied by 1.05 grams of solid silica; that is, 39.6% of the fragmented silica volume corresponds to "air in the voids within the silica heap."
Since the formulations of polymeric composites are normally gravimetric, or by weight of bulk solids and liquid fractions, and furthermore, since the entrapped air is of negligible weight, its presence is not recognized gravimetrically. However, as detailed above, the properties of polymeric composites are related primarily to volumes of the constituent solid and liquid phase fractions, which, of course, include whatever volumes are actually occupied by air and water vapor entrapped in void spaces of fragmented or filamented solids. Moreover, the air, water vapor and other gases entrained in the voids of the solids add an important contribution to the total volume of the mixed compound material when the original solids are mixed with the liquid polymeric resin system. In fact, it is important to recognize that at the start of mixing of the two primary phase polymeric compound, actually three phases are present: (1) the original primary solid phase, (2) the original primary solidifiable liquid phase (e.g., a polymeric resin system), plus (3) a gaseous phase made up of the entrained air, water vapor and other gases in the primary solid phase, plus entrained air, water vapor and other gases that may be dispersed in the resin system. Moreover, interfacially active substances generally added in the resin manufacture stabilize air inclusions.
The presence of voids in a solid composite material interferes with its integrity because voids randomly interrupt the continuity, not only of the primary solidified liquid phase, but more importantly, also the continuity of the interfacial bond between the primary phases. Void sizes, number, distribution, and especially, locations are all critical because voids determine singular points of discontinuity within the phases of the material. These discontinuities compromise the composite's integrity, strength, and further, lead to the initiation of failures due to the localized stress concentration points they create. Moreover, if these voids in the mixed unsolidified state are filled with air and water vapor, O.sub.2 in air will cause an inhibitory effect on the polymerization reaction of the resin. Water, particularly in liquid state, can be even more detrimental than O.sub.2 to the polymerization reaction and to solidification. Thus, the removal of air, water vapor and other gases from the primary liquid resin system can result in more complete polymerization/solidification of the resin, thereby producing a material with greater strength and integrity.
For example, in fiber reinforced polymer composites, voids upset the rule of mixtures. Interlaminar shear strength should increase with increasing fiber volume fraction content. Instead, shear strength actually decreases, even at relatively low void contents. Experiments show that a 5% void content causes a 35 to 40% drop in interlaminar shear strength in a fiber/epoxy composite. (Delaware Composites Encyclopedia, Vol. 1, page 29, Technomics Publishing Co. 1989). In many fiber reinforced composite fabrication processes, void content tends to increase with decreasing resin content, i.e., with increased solid content. Again, it is notable that all strength properties of fiber reinforced polymer composites drop off at higher fiber volume fractions contents- generally above 50% fiber volume content, contrary to the expectations of the rule of mixtures. A particular study for E-glass/epoxy unidirectional composite made by filament winding shows a reduction of 30% of linear fiber stress strength at failure with an increase of fiber volume fraction from 60% to 70%. (Delaware Composites Encyclopedia, Vol. 1, page 66), Technomics Publishing Co., 1989). On a weight basis, typically a 60% glass fiber volume fraction is generally attained with machine processes and represents 78% of total weight of the composite. The highest reported glass fiber volume fraction content in non-machine processed composites in the industry, such as in RTM or SCRIMP, is generally about 70% by weight, which is equivalent to just 50% of fiber content by volume.
In the case of aggregate polymeric composites, however, strengths follow the rule of mixtures, and properties, particularly compression strength, actually increase with increasing aggregate volume fraction content (provided that the aggregate fraction's particle size distribution is suitably graded for highest aggregate volume packing). Moreover, this increase in mechanical properties is observed in spite of the increased void content accompanying the increased aggregate volume fraction. In this case, the increased number of gas occlusions producing voids can be offset by mechanical vibration and vacuum of the mix, resulting in a somewhat degassed mix. Notwithstanding this fact, random voids remaining in aggregate polymeric composites also constitute points of stress concentration which are detrimental to mechanical strength properties of the material and contribute significantly to the variability of properties exhibited by the final composite materials.
E. Degassing Devices
The present state of the art attempts to deal with the removal of the entrained gaseous phase after the mixing the two primary phases. To deal with gas occlusions, conventional fabrication processes of polymeric composites generally require that the viscous compound mix, with or without special air release additives, be degassed under vacuum and/or pressurized and, in some cases, also mechanically agitated, vibrated, compacted, or combinations thereof. The application of these process steps enables movement of the gaseous phase within the viscous liquid mix assisting it to migrate towards the external surfaces of the liquid mass, escaping outside into the surrounding space. The freed gases then can be extracted by vacuum. Essentially, in the prior art, the gaseous phase is brought into the mixture entrained by the solids and/or by the liquid resin system and gets dispersed into the mixed unsolidified state. Therefore , in order to allow complete wet-out of the solids by the liquid resin system, some mechanism for removal of the gaseous phase is required. This is generally accomplished by degassing thin films of the mix under vacuum, which allows the occluded gas bubbles within the thin section to move towards its external surface. Moreover, these external surfaces are maintained at a lower pressure than the thin mass itself, thus facilitating evacuation by vacuum.
Present state of the art phase-mixing processes used to process polymeric composites, however, are not designed, nor intended, to eliminate entrained air, water vapor and other gases in the solids and liquid phases prior to the mixing process. Generally, the prior art methods of degassing are designed to work with the untreated primary phases already in the mixed state. Evacuation of gases from the mix is not only more inefficient and difficult but also less effective. Moreover, the mix can only be partially evacuated through mechanical and vacuum methods. Thus, the presence of voids in the final composite is inevitable using prior art methods.
For example, application of vacuum in a fiber polymeric composite made in a typical RTM or SCRIMP process does diminish entrapped air within the closed mold, or system, and from the glass fiber materials. Also gas vapors from the constituents of the resin system or from entrained air can be diminished by the application of vacuum, as evidenced by the reduction of visible occlusions in the solidified two-phase material. Likewise, vacuum applied to the thin sections of aggregate polymeric compounds in mixed state, especially in conjunction with mechanical vibration, which allows entrained air to be dislodged, and with air release additives that reduce interfacial tension, diminishes the total entrapped air and gases, and consequently, substantially diminishes occlusions/voids in the solidified two-phase material.
In particular cases, such as aggregate polymeric compounds involving resin/small diameter particulate microfiller mixtures, degassing a thin film of this mix with high vacuum, as described in U.S. Pat. No. 5,534,047, results in substantial elimination of gases from the mix and accordingly, a substantially "void free" composite is obtained. The success of this method is largely due to the microsize range of the filler in the primary solid phase and to relatively high resin matrix fraction volume of low viscosity, which allow good homogeneous dispersion of the solids in the liquid resin matrix. In this case, the primary solidifiable liquid phase resembles a low viscosity liquid, and therefore, behaves more like a pure liquid system. However, degassing by thin film vacuum as suggested in U.S. Pat. No. 5,534,047, is limited to a narrow range of applications. These applications involve either low viscosity liquids or low or moderate viscosity solid/liquid mixes that are capable of uniform gravity flow as thin films over flat surfaces, and that allow for relatively unimpeded movement of entrained gas occlusions by pressure differential through the viscous liquid film. Generally, however, in the prior art, two primary phase polymeric compounds are known to be incapable of being completely degassed by conventional methods including high vacuum.
F. Conclusion
Overall then, it appears that conventional processes as practiced in the present art of producing polymeric compounds cannot completely eliminate gas occlusions and voids from the compounds, and accordingly, from the corresponding solidified composites thus obtained from them. Polymeric composites produced in the prior art, therefore exhibit, high variability as well as decreased mechanical and physical properties as compared with the expected capabilities and performance of final composites produced ideally void free Moreover, the apparent acceptance in the composites industry of the presence of voids as unavoidable in the production of polymeric composite materials has precluded their potential cost effective penetration into new more technically demanding applications.
Polymeric resin system materials cost is one of the major factors affecting overall composite costs. Efforts to decrease resin system cost for increased composites competitiveness in market penetration have been generally frustrated because associated increases in solid content generally worsen rather than improve mechanical, physical and chemical properties, while significantly increasing production difficulty.
Thus, there exists a need to produce polymeric composites-both in fiber and aggregate classes-meeting a stricter and more rigorous criterion regarding freedom from gas occlusions and voids. If void-free, two-phase solid polymeric composite materials can be readily produced , they will at least exhibit increased mechanical, chemical resistance and physical strength, decreased variability of properties and enhanced reliability and performance. Polymeric composites in such a void-free solid state condition would both lower costs and improve quality in existing applications and thus, enable cost effective access to new, more demanding applications.